We have studied single stranded (ss) RNA binding immunoglobulins in the sera of patients with systemic lupus erythematosus as part of a project to obtain antibodies with conformational specificity for nucleic acids. The antibody population that bound polyribonucleotides and the population that bound polydeoxyribonucleotides appear to be distinct. The anti ss-RNA antibodies preferentially bound native ssRNA (as ccmpared with synthetic polyribonucleotides) with an increasing affinity for the higher molecular weight RNA's. We have also developed an assay for detection of specific RNA antibodies produced in vitro by spleen cells from old female NZB/NZW F1 mice. This assay enables one to test the effect of different therapeutic agents at the immediate site of antibody synthesis. BIBLIOGRAPHIC REFERENCE: Eilat, D., Di Natale, P., Steinberg, A.D., and Schechter, A.N.: Properties of tRNA Specific Antibodies from NZB/NZW Mice. J. Immuno. 118, 1016-1024 (1977).